Methods of ionization without cleaving molecular compounds, such as a laser desorption/ionization method (LDI) and a matrix-assisted laser desorption/ionization method (MALDI), are put into practical use, and mass spectrometry is widely used particularly in a field of life science. Furthermore, mass-spectrum imaging, which is a method of two-dimensionally deriving a mass spectrum based on mass spectrometry while changing an irradiating position of a laser, is drawing attention. A secondary ion mass spectrometry (SIMS) has advantages because sample preparation is simple and high lateral resolution can be carried out for a converged primary ions beam. The SIMS is an ionization method for ionizing atoms and molecules by irradiating a sample with primary ions. A time-of-flight (TOF) mass spectrometer, which has wide measurement range, is often combined as an analyzer with SIMS and is used as a time-of-flight secondary ion spectrometry (TOF-SIMS).
The SIMS is classified into dynamic-SIMS and static-SIMS based on the difference in the amount of primary ions. In the dynamic-SIMS, a large amount of secondary ions is generated by sputtering a surface of a sample irradiating a large amount of primary ions. Meanwhile, in the static-SIMS, secondary ions, the molecular structure of which is maintained, are generated because the amount of primary ions is sufficiently smaller than the number of constituent elements of the surface. Plenty of information related to the molecular structure can be derived by the static-SIMS. Therefore, the static-SIMS is more advantageous than the dynamic-SIMS in the composition analysis of organic matters. Thus, the static-SIMS is usually used in the TOF-SIMS.
A pulsed laser is necessary to use a laser as a primary beam and a TOF detector as an analyzer. Therefore, a nitrogen laser that can generate a short pulse of about 100 ps width is favorably used.
The TOF-SIMS is usually equipped with a beam-bunching mechanism to realize high mass resolution. The beam bunching is a mechanism for compressing the pulsed beam to reduce the pulse width of the primary ion beam irradiating the sample. Thus, short-pulsed primary ions irradiate the surface of the sample at the same time. As a result, high secondary ion intensity is derived, and the mass resolution improves.
However, the speed of the primary ion beam widely varies if the beam bunching is performed. As a result, the beam diameter becomes large. In general, the primary-ion-beam irradiation system includes an ion source, a pulsing mechanism, a beam-bunching mechanism, and a converging lens. If the speed (or energy) of the primary ion beam widely varies due to the beam bunching, the effect of the chromatic aberration generated by a converging lens becomes large, and the convergence is insufficient. When the beam-bunching mechanism is implemented, the diameter of the primary ion beam irradiating the sample is usually about 2 μm. If the beam diameter of the primary ions beam becomes large, the spatial resolution of the derived two-dimensional image is reduced.
FIGS. 2A and 2B are schematic diagrams illustrating irradiation intervals of the pulsed primary ions and ion-generated areas. In the TOF-SIMS, the magnification of the mass-spectrum imaging is determined by a horizontal scanning interval 1 and a vertical scanning interval 2 of the primary ion beam. The secondary ions are generated from an irradiation area 3 of the primary ion beam. Therefore, secondary ion-generated areas do not overlap in low-magnification observation, because the irradiation intervals are sufficiently wide as illustrated in FIG. 2A. However, the irradiation intervals are narrow in high-magnification observation as illustrated in FIG. 2B. More specifically, the secondary ion-generated areas gradually start to overlap as the magnification is increased, and information of secondary ions generated from a substance at an adjacent position starts to be mixed. As a result, the spatial resolution of the mass-spectrum image is reduced.
Two methods can be considered to solve this.
One method is to correct a primary-ion-beam irradiation optical system to converge the ion beam. PTL 1 discloses a technique of using a correction lens to remove the influence of the spherical aberration and the chromatic aberration of the lens to converge the ion beam. However, the speed of the ion beam widely varies in the TOF-SIMS, and the correction-lens technique illustrated in a conventional example described in PTL 1 cannot be used as it is.
The other method is to restore a derived image. In general, image blur occurs to an image derived using an apparatus depending on an observation apparatus and an observation target. A method of removing the blur from the blurred image to obtain a clear image is widely known not only in a microscopic field, but also in telescopic and signal-processing fields. In such a case, an image-restoration algorithm is used. More specifically, the cause of the image blur is handled as a function to execute numerical processing to reduce the image blur. The function defining the cause of the image blur is called a “blurring function”. PTL 2 discloses an image restoration of measuring spatial spread of the laser beam irradiating the surface of the sample in a scanning laser microscope and using the spatial spread as a blurring function. However, the measurement of the shape is difficult in the static-SIMS, because the intensity of the primary ion beam is weak. Therefore, it is difficult to measure the spatial spread and intensity of the beam as illustrated in the conventional example described in PTL 2 to use the spatial spread and intensity as a blurring function.